[Elecraft] Balanced AT

[Geoffrey Mackenzie-Kennedy]

Jack Brindle <jackbrindle at earthlink.net> wrote on Sunday, September 09, 2007
at 4:15 AM :

> While such a tuner might be fun to work with, it would unfortunately  have 
> a very short life due to burnout of the relay contacts when  tuning at 
> full power. There are also very high voltages created for  various 
> combinations of L and C while tuning is going on, which would  undoubtedly 
> shorten the life of the other components.

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Hot switching can be / usually is a recipe for disaster as you point out.
The trick is to design the system so that the tuning of the antenna matching
unit (ATU) is done at milliwatt power levels, which also helps to reduce RF
pollution. Methods to achieve band switching or coarse tuning of the remote
AMU, depending on the type used, include the use of either DC comparators or
frequency selective switches and no doubt simple digital schemes, but I have
not used digital. Frequency selective switches do require a shot of a few
milliwatts of RF input to work of course.

Keeping the size of any control cabling between the shack and remote unit to
a minimum is a personal requirement. My main feeder, control cable for a
remote 5 way coax switch and 3 wire cable for the remote AMU used with a
reference doublet all run together underground in plastic pipe.

73,
Geoff
GM4ESD